Mutations in LTBP4 cause a syndrome of impaired pulmonary, gastrointestinal, genitourinary, musculoskeletal, and dermal development

Material properties of the embryonic small intestine during buckling morphogenesis

During embryonic development, tissues undergo dramatic deformations as functional morphologies are stereotypically sculpted from simple rudiments. Formation of healthy, functional organs therefore requires tight control over the material properties of embryonic tissues during development, yet the biological basis of embryonic tissue mechanics is poorly understood. The present study investigates the mechanics of the embryonic small intestine, a tissue that is compactly organized in the body cavity by a mechanical instability during development, wherein differential elongation rates between the intestinal tube and its attached mesentery create compressive forces that buckle the tube into loops. The wavelength and curvature of these loops are tightly conserved for a given species. Focusing on the intestinal tube, we combined micromechanical testing with histologic analyses and enzymatic degradation experiments to conclude that elastic fibers closely associated with intestinal smooth muscle layers are responsible for the bending stiffness of the tube, and for establishing its pronounced mechanical anisotropy. These findings provide insights into the developmental role of elastic fibers in controlling tissue stiffness, and raise new questions on the physiologic function of elastic fibers in the intestine during adulthood. STATEMENT OF SIGNIFICANCE: The functional form of adult organs is established during embryogenesis through the action of physical forces on tissues with precise material properties. Despite this, however, biological control of material properties during embryogenesis is poorly understood. Focusing on the small intestine, we identified elastic fibers - rather than oriented smooth muscle - as defining bending stiffness, prescribing the lengthy intestine to be buckled precisely into compact loops for proper placement within the body cavity. We revealed a role for elastin in storing elastic energy during cell contraction, highlighting a potential role for elastin in gut motility through the ability to resist cyclic deformations associated with peristalsis. These results provide insights into intestinal development and adult function, and highlight elastin's diverse roles during organogenesis.

Fibulin-4 and latent-transforming growth factor beta-binding protein-4 interactions with syndecan-2 and syndecan-3 are required for elastogenesis

Elastogenesis is a cell surface-located hierarchical process that requires the core components tropoelastin and fibrillins and several accessory proteins, including fibulin-4 (FBLN4) and latent TGF-β binding protein-4 (LTBP4). FBLN4 and LTBP4 interact with cells, but their cell receptors and associated molecular elastogenic mechanisms remain unknown. Primary skin fibroblasts and several vascular smooth muscle cells bound strongly to FBLN4 multimers and LTBP4 monomers. We identified two cell interaction epitopes on FBLN4 located in cbEGF2-3 and the C-terminal domain, whereas FBLN4 multimerization sites were mapped to cbEGF4-5 and the C-terminal domain. We also determined a novel cell interaction site in the N-terminal half of LTBP4. Cell binding to FBLN4 and LTBP4 was strongly inhibited in the presence of heparin, heparan sulfate, or after enzymatic removal of heparan sulfate, suggesting heparan sulfate proteoglycans as relevant cell surface receptors. siRNA knockdown experiments identified syndecan (SDC)2 and SDC3 as cell receptors for FBNL4 and SDC3 for LTBP4. Direct protein interactions between FBLN4 and the recombinant ectodomains of SDC2 and SDC3, and between LTBP4 and SDC3 validated these results. Interaction of the elastogenic cells with FBLN4 and LTBP4 enhanced elastogenesis, whereas SDC2 and/or SDC3 knockdowns led to reduced elastic fiber formation. The cell interactions with FBLN4 and LTBP4 significantly enhanced focal adhesion formation, induced cell contraction, and led to activation of focal adhesion kinase (FAK), Erk1/2, and RhoA. Pharmacological inhibition of these effectors markedly attenuated elastic fiber formation, and siRNA knockdown of SDC2 and SDC3 led to reduced levels of pFAK, pERK, and active RhoA. Together, these data demonstrate that FBLN4 and LTBP4 cell interactions through SDC2 and SDC3 promote elastogenesis by enhancing focal adhesion formation, leading to cell contractility through FAK, Erk1/2, and RhoA activation, underscoring the significance of these pathways in elastogenesis.

Congenital cutis laxa type IC in a newborn with a newly identified genetic variant

Congenital cutis laxa is a rare connective tissue disorder, often apparent at birth or shortly thereafter, characterised by loose, redundant and inelastic skin that hangs in folds. In addition to cutaneous abnormalities, the condition can have systemic features including pulmonary, cardiovascular and gastrointestinal involvement, with varying severity depending on the underlying genetic mutation. We report the case of a Caucasian male newborn who presented with loose, wrinkled skin at birth. Initially evaluated for both cardiac and diaphragmatic abnormalities, the dermatological assessment was crucial in establishing the diagnosis, which was later confirmed by genetic testing. This case highlights the importance of early recognition and comprehensive assessment of cutaneous and systemic manifestations in newborns with suspected connective tissue disorders.

Trabecular Meshwork Abnormalities in a Model of Congenital Glaucoma Due to LTBP2 Mutation

Purpose

To characterize early trabecular meshwork (TM) morphologic abnormalities in a feline model of human primary congenital glaucoma (PCG) caused by mutation in LTBP2.

Methods

Eyes from 41 cats, including 19 normal and 22 homozygous for LTBP2 mutation, across various postnatal stages (birth, 2 weeks, 5 weeks, and 12 weeks) were paraformaldehyde fixed, anterior segments dissected, post-fixed in glutaraldehyde, osmicated, and processed and sectioned for transmission electron microscopy. Cell morphology, nuclear shape, and intertrabecular space (ITS) were quantitatively assessed, and the structure of the fibrillar extracellular matrix in the TM was systematically evaluated.

Results

The earliest differences in TM morphology between PCG and normal cats were identified at 2 weeks postnatally. Elastic fibers in the TM were discontinuous and disorganized (P = 0.0122), and by 5 weeks of age PCG cats presented significantly less ITS (P = 0.0076) and morphologically rounder TM cells than normal cats (P = 0.0293). By 12 weeks of age, the ITS was further collapsed (P < 0.0001), and the TM cells were morphologically elongated and attenuated in PCG compared to controls (P = 0.0028).

Conclusions

In this feline model of PCG due to LTBP2 mutation, development of ultrastructural TM extracellular matrix abnormalities are first observed by 2 weeks and cellular abnormalities by 5 weeks of age. By 12 weeks of age, when intraocular pressure becomes significantly elevated, the TM morphologic abnormalities are already well established. These findings suggest that the postnatal period between 0 and 5 weeks of age is critical for TM and PCG development and progression in cats.

Role of genetics and the environment in the etiology of congenital diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) is a congenital malformation characterized by failure of diaphragm closure during embryonic development, leading to pulmonary hypoplasia and pulmonary hypertension, which contribute significantly to morbidity and mortality. The occurrence of CDH and pulmonary hypoplasia is theorized to result from both abnormalities in signaling pathways of smooth muscle cells in pleuroperitoneal folds and mechanical compression by abdominal organs within the chest cavity on the developing lungs. Although, the precise etiology of diaphragm maldevelopment in CDH is not fully understood, it is believed that interplay between genes and the environment contributes to its onset. Approximately 30% of patients with CDH possess chromosomal or single gene defects and these patients tend to have inferior outcomes compared with those without genetic associations. At present, approximately 150 gene variants have been linked to the occurrence of CDH. The variable expression of the CDH phenotype in the presence of a recognized genetic predisposition can be explained by an environmental effect on gene penetrance and expression. The retinoic acid pathway is thought to play an essential role in the interactions of genes and environment in CDH. However, apart from the gradually maturing retinol hypothesis, there is limited evidence implicating other environmental factors in CDH occurrence. This review aims to describe the pathogenesis of CDH by summarizing the genetic defects and potential environmental influences on CDH development.

MATERIAL PROPERTIES OF THE EMBRYONIC SMALL INTESTINE DURING BUCKLING MORPHOGENESIS

During embryonic development, tissues undergo dramatic deformations as functional morphologies are stereotypically sculpted from simple rudiments. Formation of healthy, functional organs therefore requires tight control over the material properties of embryonic tissues during development, yet the biological basis of embryonic tissue mechanics is poorly understood. The present study investigates the mechanics of the embryonic small intestine, a tissue that is compactly organized in the body cavity by a mechanical instability during development, wherein differential elongation rates between the intestinal tube and its attached mesentery create compressive forces that buckle the tube into loops with wavelength and curvature that are tightly conserved for a given species. Focusing on the intestinal tube, we combined micromechanical testing with histologic analyses and enzymatic degradation experiments to conclude that elastic fibers closely associated with intestinal smooth muscle layers are responsible for the bending stiffness of the tube, and for establishing its pronounced mechanical anisotropy. These findings provide insights into the developmental role of elastic fibers in controlling tissue stiffness, and raise new questions on the physiologic function of elastic fibers in the intestine during adulthood.

Bilateral lung transplantation for pulmonary emphysema associated with cutis laxa

Cutis laxa is a rare elastic tissue disorder that mainly affects the skin and results in loss of elasticity. Occasionally, pulmonary emphysema complicates this condition. Herein, we report the first case of successful lung transplantation for severe juvenile emphysema associated with cutis laxa. The patient underwent bilateral deceased-donor lung transplantation. The patient's lungs had very thin visceral pleura due to a deficiency of elastic fibers along with bronchomalacia and fragile bronchial cartilage. The right upper bronchus and truncus intermedius were individually anastomosed to improve bronchial healing. At the 6-month follow-up, bronchoscopy revealed successful healing of all bronchial anastomoses, and the patient's quality of life improved. Lung transplantation is effective for treating pulmonary emphysema caused by cutis laxa.

Impact on the Transcriptome of Proton Beam Irradiation Targeted at Healthy Cardiac Tissue of Mice

Proton beam therapy is considered a step forward with respect to electromagnetic radiation, thanks to the reduction in the dose delivered. Among unwanted effects to healthy tissue, cardiovascular complications are a known long-term radiotherapy complication. The transcriptional response of cardiac tissue from xenografted BALB/c nude mice obtained at 3 and 10 days after proton irradiation covering both the tumor region and the underlying healthy tissue was analyzed as a function of dose and time. Three doses were used: 2 Gy, 6 Gy, and 9 Gy. The intermediate dose had caused the greatest impact at 3 days after irradiation: at 2 Gy, 219 genes were differently expressed, many of them represented by zinc finger proteins; at 6 Gy, there were 1109, with a predominance of genes involved in energy metabolism and responses to stimuli; and at 9 Gy, there were 105, mainly represented by zinc finger proteins and molecules involved in the regulation of cardiac function. After 10 days, no significant effects were detected, suggesting that cellular repair mechanisms had defused the potential alterations in gene expression. The nonlinear dose-response curve indicates a need to update the models built on photons to improve accuracy in health risk prediction. Our data also suggest a possible role for zinc finger protein genes as markers of proton therapy efficacy.

Multi-organ phenotypes in mice lacking latent TGFβ binding protein 2 (LTBP2)

BACKGROUND

Latent TGFβ binding protein-2 (LTBP2) is a fibrillin 1 binding component of the microfibril. LTBP2 is the only LTBP protein that does not bind any isoforms of TGFβ, although it may interfere with the function of other LTBPs or interact with other signaling pathways.

RESULTS

Here, we investigate mice lacking Ltbp2 (Ltbp2-/- ) and identify multiple phenotypes that impact bodyweight and fat mass, and affect bone and skin development. The alterations in skin and bone development are particularly noteworthy since the strength of these tissues is differentially affected by loss of Ltbp2. Interestingly, some tissues that express high levels of Ltbp2, such as the aorta and lung, do not have a developmental or homeostatic phenotype.

CONCLUSIONS

Analysis of these mice show that LTBP2 has complex effects on development through direct effects on the extracellular matrix (ECM) or on signaling pathways that are known to regulate the ECM.

Extracellular matrix in skin diseases: The road to new therapies

BACKGROUND

The extracellular matrix (ECM) is a vital structure with a dynamic and complex organization that plays an essential role in tissue homeostasis. In the skin, the ECM is arranged into two types of compartments: interstitial dermal matrix and basement membrane (BM). All evidence in the literature supports the notion that direct dysregulation of the composition, abundance or structure of one of these types of ECM, or indirect modifications in proteins that interact with them is linked to a wide range of human skin pathologies, including hereditary, autoimmune, and neoplastic diseases. Even though the ECM's key role in these pathologies has been widely documented, its potential as a therapeutic target has been overlooked.

AIM OF REVIEW

This review discusses the molecular mechanisms involved in three groups of skin ECM-related diseases - genetic, autoimmune, and neoplastic - and the recent therapeutic progress and opportunities targeting ECM.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This article describes the implications of alterations in ECM components and in BM-associated molecules that are determinant for guaranteeing its function in different skin disorders. Also, ongoing clinical trials on ECM-targeted therapies are discussed together with future opportunities that may open new avenues for treating ECM-associated skin diseases.

LTBP4 Protects Against Renal Fibrosis via Mitochondrial and Vascular Impacts

BACKGROUND

As a part of natural disease progression, acute kidney injury (AKI) can develop into chronic kidney disease via renal fibrosis and inflammation. LTBP4 (latent transforming growth factor beta binding protein 4) regulates transforming growth factor beta, which plays a role in renal fibrosis pathogenesis. We previously investigated the role of LTBP4 in chronic kidney disease. Here, we examined the role of LTBP4 in AKI.

METHODS

LTBP4 expression was evaluated in human renal tissues, obtained from healthy individuals and patients with AKI, using immunohistochemistry. LTBP4 was knocked down in both C57BL/6 mice and human renal proximal tubular cell line HK-2. AKI was induced in mice and HK-2 cells using ischemia-reperfusion injury and hypoxia, respectively. Mitochondrial division inhibitor 1, an inhibitor of DRP1 (dynamin-related protein 1), was used to reduce mitochondrial fragmentation. Gene and protein expression were then examined to assess inflammation and fibrosis. The results of bioenergetic studies for mitochondrial function, oxidative stress, and angiogenesis were assessed.

RESULTS

LTBP4 expression was upregulated in the renal tissues of patients with AKI. Ltbp4-knockdown mice showed increased renal tissue injury and mitochondrial fragmentation after ischemia-reperfusion injury, as well as increased inflammation, oxidative stress, and fibrosis, and decreased angiogenesis. in vitro studies using HK-2 cells revealed similar results. The energy profiles of Ltbp4-deficient mice and LTBP4-deficient HK-2 cells indicated decreased ATP production. LTBP4-deficient HK-2 cells exhibited decreased mitochondrial respiration and glycolysis. Human aortic endothelial cells and human umbilical vein endothelial cells exhibited decreased angiogenesis when treated with LTBP4-knockdown conditioned media. Mitochondrial division inhibitor 1 treatment ameliorated inflammation, oxidative stress, and fibrosis in mice and decreased inflammation and oxidative stress in HK-2 cells.

CONCLUSIONS

Our study is the first to demonstrate that LTBP4 deficiency increases AKI severity, consequently leading to chronic kidney disease. Potential therapies focusing on LTBP4-associated angiogenesis and LTBP4-regulated DRP1-dependent mitochondrial division are relevant to renal injury.

Specific Overexpression of YAP in Vascular Smooth Muscle Attenuated Abdominal Aortic Aneurysm Formation by Activating Elastic Fiber Assembly via LTBP4

Abdominal aortic aneurysm (AAA) is a fatal vascular disease. Vascular smooth muscle cells (VSMCs) play a crucial role in the pathogenesis of AAA. Increasing evidence has shown that Yes-associated protein (YAP) is involved in diverse vascular diseases. However, the role of YAP in AAA remains unclear. The current study aimed to determine the role of YAP in AAA formation and the underlying mechanism. We found that YAP expression in VSMCs was markedly decreased in human and experimental AAA samples. Furthermore, VSMC-specific YAP overexpression prevented several pathogenic factor-induced AAA. Mechanistically, YAP overexpression in VSMCs promoted latent transforming growth factor-β binding protein 4 (LTBP4) expression, an important factor in elastic fiber assembly. Finally, silencing of LTBP4 in VSMCs abolished the protective role of YAP in AAA formation in vivo. Our results suggest that YAP promotes LTBP4-mediated elastic fibril assembly in VSMCs, which mitigates elastin degradation and AAA formation.

First report of a short in-frame biallelic deletion removing part of the EGF-like domain calcium-binding motif in LTBP4 and causing autosomal recessive cutis laxa type 1C

Cutis laxa (CL) is a rare connective tissue disorder characterized by wrinkled, abundant and sagging skin, sometimes associated with systemic impairment. Biallelic alterations in latent transforming growth factor beta-binding protein 4 gene (LTBP4) cause autosomal recessive type 1C cutis laxa (ARCL1C, MIM #613177). The present report describes the case of a 17-months-old girl with cutis laxa together with a literature review of previous ARCL1C cases. Based on proband main clinical signs (cutis laxa and pulmonary emphysema), clinical exome sequencing (CES) was performed and showed a new nine base-pairs homozygous in-frame deletion in LTBP4 gene. RT-PCR and cDNA Sanger sequencing were performed in order to clarify its impact on RNA. This report demonstrates that a genetic alteration in the EGF-like 14 domain calcium-binding motif of LTBP4 gene is likely responsible for cutis laxa in our patient.

Latent Transforming Growth Factor β Binding Protein 3 Controls Adipogenesis

Transforming growth factor-beta (TGFβ) is released from cells as part of a trimeric latent complex consisting of TGFβ, the TGFβ propeptides, and either a latent TGFβ binding protein (LTBP) or glycoprotein-A repetitions predominant (GARP) protein. LTBP1 and 3 modulate latent TGFβ function with respect to secretion, matrix localization, and activation and, therefore, are vital for the proper function of the cytokine in a number of tissues. TGFβ modulates stem cell differentiation into adipocytes (adipogenesis), but the potential role of LTBPs in this process has not been studied. We observed that 72 h post adipogenesis initiation Ltbp1, 2, and 4 expression levels decrease by 74-84%, whereas Ltbp3 expression levels remain constant during adipogenesis. We found that LTBP3 silencing in C3H/10T1/2 cells reduced adipogenesis, as measured by the percentage of cells with lipid vesicles and the expression of the transcription factor peroxisome proliferator-activated receptor gamma (PPARγ). Lentiviral mediated expression of an Ltbp3 mRNA resistant to siRNA targeting rescued the phenotype, validating siRNA specificity. Knockdown (KD) of Ltbp3 expression in 3T3-L1, M2, and primary bone marrow stromal cells (BMSC) indicated a similar requirement for Ltbp3. Epididymal and inguinal white adipose tissue fat pad weights of Ltbp3^-/- mice were reduced by 62% and 57%, respectively, compared to wild-type mice. Inhibition of adipogenic differentiation upon LTBP3 loss is mediated by TGFβ, as TGFβ neutralizing antibody and TGFβ receptor I kinase blockade rescue the LTBP3 KD phenotype. These results indicate that LTBP3 has a TGFβ-dependent function in adipogenesis both in vitro and possibly in vivo.

Autosomal recessive cutis laxa type 1C with a homozygous LTBP4 splicing variant: a case report and update of literature

BACKGROUND

Autosomal recessive cutis laxa (ARCL) is a heterogeneous disorder with three primary forms (ARCL 1, ARCL 2 and ARCL 3). Latent transforming growth factor beta binding protein 4 (LTBP4) anomalies cause ARCL1C and are connected to different problems in the skin and other organs. Herein, we present a seven month old Iranian boy with a clinical manifestation of ARCL1 with literature review of previous cases with attributes of ARCL1C.

METHODS

Considering the craniofacial characteristics and respiratory distress of the proband, cutis laxa (CL) was expected and whole-exome sequencing (WES) was performed.

RESULTS

In the proband, signs of CL were mainly located in the face, thorax, and abdomen. The prenatal investigation revealed a diaphragmatic hernia and certain uncommon signs, such as an atrial septal defect and pyloric stenosis. The WES showed a novel homozygous mutation (c.533-1G > A) in exon six of the LTBP4 gene.

CONCLUSION

This report showed a new variant with uncommon clinical features, such as a stenosis atrial septal defect and pyloric stenosis, which causes ARCL1C. Unfortunately, the proband developed several heart problems and died at the age of seven months and seven days. Thus, a more in-depth evaluation is needed to clarify the different aspects of CL related to LTBP4 disorder.

Elastic Fibre Proteins in Elastogenesis and Wound Healing

As essential components of our connective tissues, elastic fibres give tissues such as major blood vessels, skin and the lungs their elasticity. Their formation is complex and co-ordinately regulated by multiple factors. In this review, we describe key players in elastogenesis: fibrillin-1, tropoelastin, latent TGFβ binding protein-4, and fibulin-4 and -5. We summarise their roles in elastogenesis, discuss the effect of their mutations on relevant diseases, and describe their interactions involved in forming the elastic fibre network. Moreover, we look into their roles in wound repair for a better understanding of their potential application in tissue regeneration.

Collagen XII Regulates Corneal Stromal Structure by Modulating Transforming Growth Factor-β Activity

Collagen XII is a regulator of corneal stroma structure and function. The current study examined the role of collagen XII in regulating corneal stromal transforming growth factor (TGF)-β activation and latency. Specifically, with the use of conventional collagen XII null mouse model, the role of collagen XII in the regulation of TGF-β latency and activity in vivo was investigated. Functional quantification of latent TGF-β in stromal matrix was performed by using transformed mink lung reporter cells that produce luciferase as a function of active TGF-β. Col12a1 knockdown with shRNA was used to test the role of collagen XII in TGF-β activation. Col12a1^-/- hypertrophic stromata were observed with keratocyte hyperplasia. Increased collagen fibril forward signal was found by second harmonic generation microscopy in the absence of collagen XII. Collagen XII regulated mRNA synthesis of Serpine1, Col1a1, and Col5a1 and deposition of collagens in the extracellular matrix. A functional plasminogen activator inhibitor luciferase assay showed that collagen XII is necessary for latent TGF-β storage in the extracellular matrix and that collagen XII down-regulates active TGF-β. Collagen XII dictates stromal structure and function by regulating TGF-β activity. A hypertrophic phenotype in Col12a1^-/- corneal tissue can be explained by abnormal up-regulation of TGF-β activation and decreased latent storage.

The role of LTBPs in TGF beta signaling

The purpose of this review is to discuss the transforming growth factor beta (TGFB) binding proteins (LTBP) with respect to their participation in the activity of TGFB. We first describe pertinent aspects of the biology and cell function of the LTBPs. We then summarize the physiological consequences of LTBP loss in humans and mice. Finally, we consider a number of outstanding questions relating to LTBP function.

Aberrant interaction between mutated ADAMTSL2 and LTBP4 is associated with adolescent idiopathic scoliosis

Adolescent idiopathic scoliosis (AIS) is a complex spinal structure deformity with a prevalence of 1%-3%. Genetic and hereditary factors have been associated with the etiology of AIS. However, previous studies mainly focused on common single nucleotide polymorphisms which confer modest disease risk. Recently, rare variants in FBN1 and other extracellular matrix genes have been implicated in AIS, suggesting a potential overlapping disease etiology between AIS and hereditary connective tissue disorders (HCTD). In this study, we systematically analyzed rare variants in a set of HCTD-related genes in 302 AIS patients using exome sequencing. We firstly focused on pathogenic variants based on a monogenic inheritance and identified nine disease-associated variants in FBN1, COL11A1, COL11A2 and TGFBR2. We then explored the potential interactions between variants in different genes based on the case-control statistics. We identified three ADAMTSL2-LTBP4 variant pairs in three AIS patients and none in controls. Furthermore, we revealed that the variant pairs identified in these genes could affect the interaction between ADAMTSL2 and LTBP4 and upregulate TGF-β signaling pathway in human fibroblasts. Our findings supported that the aberrant interaction between mutated ADAMTSL2 and LTBP4 was associated with AIS.

Clinical and Molecular Delineation of Cutis Laxa Syndromes: Paradigms for Homeostasis

Cutis laxa (CL) syndromes are a large and heterogeneous group of rare connective tissue disorders that share loose redundant skin as a hallmark clinical feature, which reflects dermal elastic fiber fragmentation. Both acquired and congenital-Mendelian- forms exist. Acquired forms are progressive and often preceded by inflammatory triggers in the skin, but may show systemic elastolysis. Mendelian forms are often pleiotropic in nature and classified upon systemic manifestations and mode of inheritance. Though impaired elastogenesis is a common denominator in all Mendelian forms of CL, the underlying gene defects are diverse and affect structural components of the elastic fiber or impair metabolic pathways interfering with cellular trafficking, proline synthesis, or mitochondrial functioning. In this chapter we provide a detailed overview of the clinical and molecular characteristics of the different cutis laxa types and review the latest insights on elastic fiber assembly and homeostasis from both human and animal studies.

Basic Components of Connective Tissues and Extracellular Matrix: Fibronectin, Fibrinogen, Laminin, Elastin, Fibrillins, Fibulins, Matrilins, Tenascins and Thrombospondins

Collagens are the most abundant components of the extracellular matrix (ECM) and many types of soft tissues. Elastin is another major component of certain soft tissues, such as arterial walls and ligaments. It is an insoluble polymer of the monomeric soluble precursor tropoelastin, and the main component of elastic fibers in matrix tissue where it provides elastic recoil and resilience to a variety of connective tissues, e.g., aorta and ligaments. Elastic fibers regulate activity of transforming growth factors β (TGFβ) through their association with fibrillin microfibrils. Elastin also plays a role in cell adhesion, cell migration, and has the ability to participate in cell signaling. Mutations in the elastin gene lead to cutis laxa. Many other molecules, though lower in quantity, function as essential, structural and/or functional components of the extracellular matrix in soft tissues. Some of these are reviewed in this chapter. Besides their basic structure, biochemistry and physiology, their roles in disorders of soft tissues are discussed only briefly as most chapters in this volume deal with relevant individual compounds. Fibronectin with its multidomain structure plays a role of "master organizer" in matrix assembly as it forms a bridge between cell surface receptors, e.g., integrins, and compounds such collagen, proteoglycans and other focal adhesion molecules. It also plays an essential role in the assembly of fibrillin-1 into a structured network. Though the primary role of fibrinogen is in clot formation, after conversion to fibrin by thrombin it also binds to a variety of compounds, particularly to various growth factors, and as such, fibrinogen is a player in cardiovascular and extracellular matrix physiology. Laminins contribute to the structure of the ECM and modulate cellular functions such as adhesion, differentiation, migration, stability of phenotype, and resistance towards apoptosis. Fibrillins represent the predominant core of microfibrils in elastic as well as non-elastic extracellular matrixes, and interact closely with tropoelastin and integrins. Not only do microfibrils provide structural integrity of specific organ systems, but they also provide basis for elastogenesis in elastic tissues. Fibrillin is important for the assembly of elastin into elastic fibers. Mutations in the fibrillin-1 gene are closely associated with Marfan syndrome. Latent TGFβ binding proteins (LTBPs) are included here as their structure is similar to fibrillins. Several categories of ECM components described after fibrillins are sub-classified as matricellular proteins, i.e., they are secreted into ECM, but do not provide structure. Rather they interact with cell membrane receptors, collagens, proteases, hormones and growth factors, communicating and directing cell-ECM traffic. Fibulins are tightly connected with basement membranes, elastic fibers and other components of extracellular matrix and participate in formation of elastic fibers. Matrilins have been emerging as a new group of supporting actors, and their role in connective tissue physiology and pathophysiology has not been fully characterized. Tenascins are ECM polymorphic glycoproteins found in many connective tissues in the body. Their expression is regulated by mechanical stress both during development and in adulthood. Tenascins mediate both inflammatory and fibrotic processes to enable effective tissue repair and play roles in pathogenesis of Ehlers-Danlos, heart disease, and regeneration and recovery of musculo-tendinous tissue. One of the roles of thrombospondin 1 is activation of TGFβ. Increased expression of thrombospondin and TGFβ activity was observed in fibrotic skin disorders such as keloids and scleroderma. Cartilage oligomeric matrix protein (COMP) or thrombospondin-5 is primarily present in the cartilage. High levels of COMP are present in fibrotic scars and systemic sclerosis of the skin, and in tendon, especially with physical activity, loading and post-injury. It plays a role in vascular wall remodeling and has been found in atherosclerotic plaques as well.

LTBP4 affects renal fibrosis by influencing angiogenesis and altering mitochondrial structure

Transforming growth factor beta (TGFβ) signalling regulates extracellular matrix accumulation known to be essential for the pathogenesis of renal fibrosis; latent transforming growth factor beta binding protein 4 (LTBP4) is an important regulator of TGFβ activity. To date, the regulation of LTBP4 in renal fibrosis remains unknown. Herein, we report that LTBP4 is upregulated in patients with chronic kidney disease and fibrotic mice kidneys created by unilateral ureteral obstruction (UUO). Mice lacking the short LTBP4 isoform (Ltbp4S-/-) exhibited aggravated tubular interstitial fibrosis (TIF) after UUO, indicating that LTBP4 potentially protects against TIF. Transcriptomic analysis of human proximal tubule cells overexpressing LTBP4 revealed that LTBP4 influences angiogenic pathways; moreover, these cells preserved better mitochondrial respiratory functions and expressed higher vascular endothelial growth factor A (VEGFA) compared to wild-type cells under hypoxia. Results of the tube formation assay revealed that additional LTBP4 in human umbilical vein endothelial cell supernatant stimulates angiogenesis with upregulated vascular endothelial growth factor receptors (VEGFRs). In vivo, aberrant angiogenesis, abnormal mitochondrial morphology and enhanced oxidative stress were observed in Ltbp4S-/- mice after UUO. These results reveal novel molecular functions of LTBP4 stimulating angiogenesis and potentially impacting mitochondrial structure and function. Collectively, our findings indicate that LTBP4 protects against disease progression and may be of therapeutic use in renal fibrosis.

Autosomal Recessive Cutis Laxa 1C Mutations Disrupt the Structure and Interactions of Latent TGFβ Binding Protein-4

Latent TGFβ binding protein-4 (LTBP4) is a multi-domain glycoprotein, essential for regulating the extracellular bioavailability of TGFβ and assembly of elastic fibre proteins, fibrillin-1 and tropoelastin. LTBP4 mutations are linked to autosomal recessive cutis laxa type 1C (ARCL1C), a rare congenital disease characterised by high mortality and severely disrupted connective tissues. Despite the importance of LTBP4, the structure and molecular consequences of disease mutations are unknown. Therefore, we analysed the structural and functional consequences of three ARCL1C causing point mutations which effect highly conserved cysteine residues. Our structural and biophysical data show that the LTBP4 N- and C-terminal regions are monomeric in solution and adopt extended conformations with the mutations resulting in subtle changes to their conformation. Similar to LTBP1, the N-terminal region is relatively inflexible, whereas the C-terminal region is flexible. Interaction studies show that one C-terminal mutation slightly decreases binding to fibrillin-1. We also found that the LTBP4 C-terminal region directly interacts with tropoelastin which is perturbed by both C-terminal ARCL1C mutations, whereas an N-terminal mutation increased binding to fibulin-4 but did not affect the interaction with heparan sulphate. Our results suggest that LTBP4 mutations contribute to ARCL1C by disrupting the structure and interactions of LTBP4 which are essential for elastogenesis in a range of mammalian connective tissues.

The Role of De Novo Variants in Patients with Congenital Diaphragmatic Hernia

The genetic etiology of congenital diaphragmatic hernia (CDH), a common and severe birth defect, is still incompletely understood. Chromosomal aneuploidies, copy number variations (CNVs), and variants in a large panel of CDH-associated genes, both de novo and inherited, have been described. Due to impaired reproductive fitness, especially of syndromic CDH patients, and still significant mortality rates, the contribution of de novo variants to the genetic background of CDH is assumed to be high. This assumption is supported by the relatively low recurrence rate among siblings. Advantages in high-throughput genome-wide genotyping and sequencing methods have recently facilitated the detection of de novo variants in CDH. This review gives an overview of the known de novo disease-causing variants in CDH patients.

Latent TGFβ-binding proteins regulate UCP1 expression and function via TGFβ2

Objective

Activation of brown adipose tissue (BAT) in humans has been proposed as a new treatment approach for combating obesity and its associated diseases, as BAT participates in the regulation of energy homeostasis as well as glucose and lipid metabolism. Genetic contributors driving brown adipogenesis in humans have not been fully understood.

Methods

Profiling the gene expression of progenitor cells from subcutaneous and deep neck adipose tissue, we discovered new secreted factors with potential regulatory roles in white and brown adipogenesis. Among these, members of the latent transforming growth factor beta-binding protein (LTBP) family were highly expressed in brown compared to white adipocyte progenitor cells, suggesting that these proteins are capable of promoting brown adipogenesis. To investigate this potential, we used CRISPR/Cas9 to generate LTBP-deficient human preadipocytes.

Results

We demonstrate that LTBP2 and LTBP3 deficiency does not affect adipogenic differentiation, but diminishes UCP1 expression and function in the obtained mature adipocytes. We further show that these effects are dependent on TGFβ2 but not TGFβ1 signaling: TGFβ2 deficiency decreases adipocyte UCP1 expression, whereas TGFβ2 treatment increases it. The activity of the LTBP3–TGFβ2 axis that we delineate herein also significantly correlates with UCP1 expression in human white adipose tissue (WAT), suggesting an important role in regulating WAT browning as well.

Conclusions

These results provide evidence that LTBP3, via TGFβ2, plays an important role in promoting brown adipogenesis by modulating UCP1 expression and mitochondrial oxygen consumption.

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Inhibition of LTBP2 and LTBP3 reduces secretion of TGFβ2.

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Both knockout of LTBP2/3 or TGFβ2 inhibit UCP1 expression and mitochondrial respiration in human adipocytes.

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Expression of TGFβ2 correlates with UCP1 expression in human adipose tissue.

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Treatment with TGFβ2 rescues inhibition of UCP1 by LTBP knockout during adipogenesis.

LTBP4 in Health and Disease

Latent transforming growth factor β (TGFβ)-binding protein (LTBP) 4, a member of the LTBP family, shows structural homology with fibrillins. Both these protein types are characterized by calcium-binding epidermal growth factor-like repeats interspersed with 8-cysteine domains. Based on its domain composition and distribution, LTBP4 is thought to adopt an extended structure, facilitating the linear deposition of tropoelastin onto microfibrils. In humans, mutations in LTBP4 result in autosomal recessive cutis laxa type 1C, characterized by redundant skin, pulmonary emphysema, and valvular heart disease. LTBP4 is an essential regulator of TGFβ signaling and is related to development, immunity, injury repair, and diseases, playing a central role in regulating inflammation, fibrosis, and cancer progression. In this review, we focus on medical disorders or diseases that may be manipulated by LTBP4 in order to enhance the understanding of this protein.

Elastic fibers: formation, function, and fate during aging and disease

Elastic fibers are extracellular components of higher vertebrates and confer elasticity and resilience to numerous tissues and organs such as large blood vessels, lungs, and skin. Their formation and maturation take place in a complex multistage process called elastogenesis. It requires interactions between very different proteins but also other molecules and leads to the deposition and crosslinking of elastin's precursor on a scaffold of fibrillin-rich microfibrils. Mature fibers are exceptionally resistant to most influences and, under healthy conditions, retain their biomechanical function over the life of the organism. However, due to their longevity, they accumulate damages during aging. These are caused by proteolytic degradation, formation of advanced glycation end products, calcification, oxidative damage, aspartic acid racemization, lipid accumulation, carbamylation, and mechanical fatigue. The resulting changes can lead to diminution or complete loss of elastic fiber function and ultimately affect morbidity and mortality. Particularly, the production of elastokines has been clearly shown to influence several life-threatening diseases. Moreover, the structure, distribution, and abundance of elastic fibers are directly or indirectly influenced by a variety of inherited pathological conditions, which mainly affect organs and tissues such as skin, lungs, or the cardiovascular system. A distinction can be made between microfibril-related inherited diseases that are the result of mutations in diverse microfibril genes and indirectly affect elastogenesis, and elastinopathies that are linked to changes in the elastin gene. This review gives an overview on the formation, structure, and function of elastic fibers and their fate over the human lifespan in health and disease.

Elastic fibers during aging and disease

Elastic fibers are essential constituents of the extracellular matrix of higher vertebrates and endow several tissues and organs including lungs, skin and blood vessels with elasticity and resilience. During the human lifespan, elastic fibers are exposed to a variety of enzymatic, chemical and biophysical influences, and accumulate damage due to their low turnover. Aging of elastin and elastic fibers involves enzymatic degradation, oxidative damage, glycation, calcification, aspartic acid racemization, binding of lipids and lipid peroxidation products, carbamylation and mechanical fatigue. These processes can trigger an impairment or loss of elastic fiber function and are associated with severe pathologies. There are different inherited or acquired pathological conditions, which influence the structure and function of elastic fibers and microfibrils predominantly in the cardiorespiratory system and skin. Inherited elastic-fiber pathologies have a direct or indirect impact on elastic-fiber formation due to mutations in the fibrillin genes (fibrillinopathies), in the elastin gene (elastinopathies) or in genes encoding proteins that are associated with microfibrils or elastic fibers. Acquired elastic-fiber pathologies appear age-related or as a result of multiple factors impairing tissue homeostasis. This review gives an overview on the fate of elastic fibers over the human lifespan in health and disease.

Tropoelastin and Elastin Assembly

Elastic fibers are an important component of the extracellular matrix, providing stretch, resilience, and cell interactivity to a broad range of elastic tissues. Elastin makes up the majority of elastic fibers and is formed by the hierarchical assembly of its monomer, tropoelastin. Our understanding of key aspects of the assembly process have been unclear due to the intrinsic properties of elastin and tropoelastin that render them difficult to study. This review focuses on recent developments that have shaped our current knowledge of elastin assembly through understanding the relationship between tropoelastin’s structure and function.

Review of clinical and molecular variability in autosomal recessive cutis laxa 2A

ATP6V0A2-related cutis laxa, also known as autosomal recessive cutis laxa type 2A (ARCL2A), is a subtype of hereditary cutis laxa originally characterized by skin, skeletal, and neurological involvement, and a combined defect of N-glycosylation and O-glycosylation. The associated clinical spectrum subsequently expanded to a less severe phenotype dominated by cutaneous involvement. At the moment, ARCL2A was described in a few case reports and series only. An Italian adult woman ARCL2A with a phenotype restricted to skin and the two novel c.3G>C and c.1101dup ATP6V0A2 variants has been reported. A systematic literature review allowed us to identify 69 additional individuals from 64 families. Available data were scrutinized in order to describe the clinical and molecular variability of ARCL2A. About 78.3% of known variants were predicted null alleles, while 11 were missense and 2 affected noncanonical splice sites. Age at ascertainment appeared as the unique phenotypic discriminator with earlier age more commonly associated with facial dysmorphism (p .02), high/cleft palate (p .005), intellectual disability/global developmental delay (p .013), and seizures (p .024). No specific genotype-phenotype correlations were identified. This work confirmed the existence of an attenuated phenotype associated with ATP6V0A2 biallelic variants and offers an updated critique to the clinical and molecular variability of ARCL2A.

Assessing the Multivariate Relationship between the Human Infant Intestinal Exfoliated Cell Transcriptome (Exfoliome) and Microbiome in Response to Diet

Gut microbiota and the host exist in a mutualistic relationship, with the functional composition of the microbiota strongly influencing the health and well-being of the host. In addition to the standard differential expression analysis of host genes to assess the complex cross-talk between environment (diet), microbiome, and host intestinal physiology, data-driven integrative approaches are needed to identify potential biomarkers of both host genes and microbial communities that characterize these interactions. Our findings demonstrate that the complementary application of univariate differential gene expression analysis and multivariate approaches such as sparse Canonical Correlation Analysis (sCCA) and sparse Principal Components Analysis (sPCA) can be used to integrate data from both the healthy infant gut microbial community and host transcriptome (exfoliome) using stool derived exfoliated cells shed from the gut. These approaches reveal host genes and microbial functional categories related to the feeding phenotype of the infants. Our findings also confirm that combinatorial noninvasive -omic approaches provide an integrative genomics-based perspective of neonatal host-gut microbiome interactions.

Two novel compound heterozygous variants of LTBP4 in a Chinese infant with cutis laxa type IC and a review of the related literature

BACKGROUND

Autosomal recessive cutis laxa type IC (ARCL IC, MIM: #613177) results from a mutation in the LTBP4 gene (MIM: #604710) on chromosome 19q13.

CASE PRESENTATION

A 28-day-old Chinese infant with generalized cutis laxa accompanied by impaired pulmonary, gastrointestinal, genitourinary, retinal hemorrhage, abnormality of coagulation and hyperbilirubinemia was admitted to our hospital. To find out the possible causes of these symptoms, whole-exome sequencing was performed on the infant. Two novel pathogenic frame-shift variants [c.605_606delGT (p.Ser204fs * 8) and c.1719delC (p.Arg574fs * 199)] of the LTBP4 gene associated with ARCL IC were found which was later verified by Sanger sequencing. The pathogenicity of mutations was subsequently assessed by several software programs and databases. In addition, an analytical review on the clinical phenotypes of the disease previously reported in literature was performed.

CONCLUSIONS

This is the first report of a Chinese infant with ARCL IC in China due to novel pathogenic variations of LTBP4. Our study extends the cutis laxa type IC mutation spectrum as well as the phenotypes associated with the disease in different populations.

Overview of the Pulmonary Manifestations in Patients with Autosomal Recessive Cutis Laxa Type IC

Background: Autosomal recessive cutis laxa type IC (ARCL1C) is characterized by cutis laxa accompanied by pulmonary, gastrointestinal, urinary, musculoskeletal involvement caused by biallelic mutations in latent transforming growth factor-beta binding protein 4 (LTBP4) gene. The overall prognosis is poor, and most patients die in infancy because of severe pulmonary emphysema (PE). Aim: We aimed to evaluate 3 ARCL1C patients, 2 of whom are still alive and in their childhood period, from 2 unrelated families with novel LTBP4 mutations, to demonstrate the clinical variability of pulmonary involvement. Materials and Methods: Three children who were molecularly confirmed by LTBP4 sequencing analysis were comprehensively reviewed in terms of pulmonary manifestations through chest examination, lung function tests (LFTs), chest X-ray, and thorax computed tomography. Results: Family 1 (c.3740A>G LTBP4 mutation): A 5-year-old male patient with pulmonary artery stenosis (PAS) presented with persistent cough and exhibited mild restriction on LFT. Family 2 (c.2T>G LTBP4 mutation): Radiographic examinations revealed PE in a 7-year-old female patient who was operated for diaphragmatic hernia. She had recurrent bronchiolitis and pulmonary infections. LFT revealed both obstructive and restrictive pattern. Her cousin also had respiratory distress with the onset of the newborn period and died due to bilateral pneumothorax in early infancy. Conclusion: The variable severity of pulmonary findings was shown in these patients. It should also be kept in mind that there could be intrafamilial variability of systemic manifestations. Although obstructive lung disease is expected to be seen in ARLC1C patients, restrictive LFT patterns may also be detected as a result of comorbidities such as diaphragmatic hernia and PAS.

Cutis laxa: A comprehensive overview of clinical characteristics and pathophysiology

Cutis laxa (CL) syndromes comprise a rare group of multisystem disorders that share loose redundant skin folds as hallmark clinical feature. CL results from impaired elastic fiber assembly and homeostasis, and the known underlying gene defects affect different extracellular matrix proteins, intracellular trafficking, or cellular metabolism. Due to the underlying clinical and molecular heterogeneity, the diagnostic work-up of CL patients is often challenging. In this review, we provide a practical approach to the broad differential diagnosis of CL syndromes, provide an overview of the molecular pathogenesis of the different subtypes, and suggest general management guidelines.

The fibrillin microfibril/elastic fibre network: A critical extracellular supramolecular scaffold to balance skin homoeostasis

Supramolecular networks composed of fibrillins (fibrillin-1 and fibrillin-2) and associated ligands form intricate cellular microenvironments which balance skin homoeostasis and direct remodelling. Fibrillins assemble into microfibrils which are not only indispensable for conferring elasticity to the skin, but also control the bioavailability of growth factors targeted to the extracellular matrix architecture. Fibrillin microfibrils (FMF) represent the core scaffolds for elastic fibre formation, and they also decorate the surface of elastic fibres and form independent networks. In normal dermis, elastic fibres are suspended in a three-dimensional basket-like lattice of FMF intersecting basement membranes at the dermal-epidermal junction and thus conferring pliability to the skin. The importance of FMF for skin homoeostasis is illustrated by the clinical features caused by mutations in the human fibrillin genes (FBN1, FBN2), summarized as "fibrillinopathies." In skin, fibrillin mutations result in phenotypes ranging from thick, stiff and fibrotic skin to thin, lax and hyperextensible skin. The most plausible explanation for this spectrum of phenotypic outcomes is that FMF regulate growth factor signalling essential for proper growth and homoeostasis of the skin. Here, we will give an overview about the current understanding of the underlying pathomechanisms leading to fibrillin-dependent fibrosis as well as forms of cutis laxa caused by mutational inactivation of FMF-associated ligands.

Skin proteomics - analysis of the extracellular matrix in health and disease

INTRODUCTION

The skin protects the human body from external insults and regulates water and temperature homeostasis. A highly developed extracellular matrix (ECM) supports the skin and instructs its cell functions. Reduced functionality of the ECM is often associated with skin diseases that cause physical impairment and also have implications on social interactions and quality of life of affected individuals.

AREAS COVERED

With a focus on the skin ECM we discuss how mass spectrometry (MS)-based proteomic approaches first contributed to establishing skin protein inventories and then facilitated elucidation of molecular functions and disease mechanisms.

EXPERT OPINION

MS-based proteomic approaches have significantly contributed to our understanding of skin pathophysiology, but also revealed the challenges in assessing the skin ECM. The numerous posttranslational modifications of ECM proteins, like glycosylation, crosslinking, oxidation, and proteolytic maturation in disease settings can be difficult to tackle and remain understudied. Increased ease of handling of LC-MS/MS systems and automated/streamlined data analysis pipelines together with the accompanying increased usage of LC-MS/MS approaches will ensure that in the coming years MS-based proteomic approaches will continue to play a vital part in skin disease research. They will facilitate the elucidation of molecular disease mechanisms and, ultimately, identification of new druggable targets.

Identification of a Novel 19-bp Deletion Mutation in LTBP4 Using Exome Sequencing in Two Siblings with Autosomal Recessive Cutis Laxa Type 1C

Autosomal recessive type I cutis laxa is genetically heterogeneous. Biallelic mutations in latent transforming growth factor β-binding protein 4 (LTBP4; MIM*604710) lead to type 1C cutis laxa due to nonsense, frameshift, single base pair indels, or duplication mutations. In this report, we describe the first Indian family with cutis laxa as a result of a novel 19 base pair homozygous deletion leading to premature termination of short isoform LTBP-4S.

Role of fibrillin-2 in the control of TGF-β activation in tumor angiogenesis and connective tissue disorders

Fibrillins constitute a family of large extracellular glycoproteins which multimerize to form microfibrils, an important structure in the extracellular matrix. It has long been assumed that fibrillin-2 was barely present during postnatal life, but it is now clear that fibrillin-2 molecules form the structural core of microfibrils, and are masked by an outer layer of fibrillin-1. Mutations in fibrillins give rise to heritable connective tissue disorders, including Marfan syndrome and congenital contractural arachnodactyly. Fibrillins also play an important role in matrix sequestering of members of the transforming growth factor-β family, and in context of Marfan syndrome excessive TGF-β activation has been observed. TGF-β activation is highly dependent on integrin binding, including integrin αvβ8 and αvβ6, which are upregulated upon TGF-β exposure. TGF-β is also involved in tumor progression, metastasis, epithelial-to-mesenchymal transition and tumor angiogenesis. In several highly vascularized types of cancer such as hepatocellular carcinoma, a positive correlation was found between increased TGF-β plasma concentrations and tumor vascularity. Interestingly, fibrillin-1 has a higher affinity to TGF-β and, therefore, has a higher capacity to sequester TGF-β compared to fibrillin-2. The previously reported downregulation of fibrillin-1 in tumor endothelium affects the fibrillin-1/fibrillin-2 ratio in the microfibrils, exposing the normally hidden fibrillin-2. We postulate that fibrillin-2 exposure in the tumor endothelium directly stimulates tumor angiogenesis by influencing TGF-β sequestering by microfibrils, leading to a locally higher active TGF-β concentration in the tumor microenvironment. From a therapeutic perspective, fibrillin-2 might serve as a potential target for future anti-cancer therapies.

TGF-β1 - A truly transforming growth factor in fibrosis and immunity

'Jack of all trades, master of everything' is a fair label for transforming growth factor β1 (TGF-β) - a cytokine that controls our life at many levels. In the adult organism, TGF-β1 is critical for the development and maturation of immune cells, maintains immune tolerance and homeostasis, and regulates various aspects of immune responses. Following acute tissue damages, TGF-β1 becomes a master regulator of the healing process with impacts on about every cell type involved. Divergence from the tight control of TGF-β1 actions, for instance caused by chronic injury, severe trauma, or infection can tip the balance from regulated physiological to excessive pathological repair. This condition of fibrosis is characterized by accumulation and stiffening of collagenous scar tissue which impairs organ functions to the point of failure. Fibrosis and dysregulated immune responses are also a feature of cancer, in which tumor cells escape immune control partly by manipulating TGF-β1 regulation and where immune cells are excluded from the tumor by fibrotic matrix created during the stroma 'healing' response. Despite the obvious potential of TGF-β-signalling therapies, globally targeting TGF-β1 receptor, downstream pathways, or the active growth factor have proven to be extremely difficult if not impossible in systemic treatment regimes. However, TGF-β1 binding to cell receptors requires prior activation from latent complexes that are extracellularly presented on the surface of immune cells or within the extracellular matrix. These different locations have led to some divergence in the field which is often either seen from the perspective of an immunologists or a fibrosis/matrix researcher. Despite these human boundaries, there is considerable overlap between immune and tissue repair cells with respect to latent TGF-β1 presentation and activation. Moreover, the mechanisms and proteins employed by different cells and spatiotemporal control of latent TGF-β1 activation provide specificity that is amenable to drug development. This review aims at synthesizing the knowledge on TGF-β1 extracellular activation in the immune system and in fibrosis to further stimulate cross talk between the two research communities in solving the TGF-β conundrum.

Unique molecular networks: Formation and role of elastin cross-links

Elastic fibers are essential assemblies of vertebrates and confer elasticity and resilience to various organs including blood vessels, lungs, skin, and ligaments. Mature fibers, which comprise a dense and insoluble elastin core and a microfibrillar mantle, are extremely resistant toward intrinsic and extrinsic influences and maintain elastic function over the human lifespan in healthy conditions. The oxidative deamination of peptidyl lysine to peptidyl allysine in elastin's precursor tropoelastin is a crucial posttranslational step in their formation. The modification is catalyzed by members of the family of lysyl oxidases and the starting point for subsequent manifold condensation reactions that eventually lead to the highly cross-linked elastomer. This review summarizes the current understanding of the formation of cross-links within and between the monomer molecules, the molecular sites, and cross-link types involved and the pathological consequences of abnormalities in the cross-linking process.

Biallelic variants in EFEMP1 in a man with a pronounced connective tissue phenotype

Connective tissue disorders are a spectrum of diseases that affect the integrity of tissues including skin, vasculature, and joints. They are often caused by variants that disrupt genes encoding components of extracellular matrix (ECM). The fibulin glycoproteins are ECM proteins important for integrity of tissues including dermis, retina, fascia, and vasculature. The fibulin family consists of seven members (fibulins-1 to -7) and is defined by a fibulin-type domain at the C-terminus. The family is associated with human diseases, for instance a variant in FBLN1, encoding fibulin-1, is associated with synpolydactyly, while one in EFEMP1, encoding fibulin-3, causes Doyne honeycomb degeneration of the retina. Loss-of-function of fibulins-4 and -5 causes cutis laxa, while variants in fibulins-5 and -6 are associated with age-related macular degeneration. Of note, EFEMP1 is not currently associated with any connective tissue disorder. Here we show biallelic loss-of-function variants in EFEMP1 in an individual with multiple and recurrent abdominal and thoracic herniae, myopia, hypermobile joints, scoliosis, and thin translucent skin. Fibroblasts from this individual express significantly lower EFEMP1 transcript than age-matched control cells. A skin biopsy, visualised using light microscopy, showed normal structure and abundance of elastic fibres. The phenotype of this individual is remarkably similar to the Efemp1 knockout mouse model that displays multiple herniae with premature aging and scoliosis. We conclude that loss of EFEMP1 function in this individual is the cause of a connective tissue disorder with a novel combination of phenotypic features, and can perhaps explain similar, previously reported cases in the literature.

The role of fibrillin and microfibril binding proteins in elastin and elastic fibre assembly

Fibrillin is a large evolutionarily ancient extracellular glycoprotein that assembles to form beaded microfibrils which are essential components of most extracellular matrices. Fibrillin microfibrils have specific biomechanical properties to endow animal tissues with limited elasticity, a fundamental feature of the durable function of large blood vessels, skin and lungs. They also form a template for elastin deposition and provide a platform for microfibril-elastin binding proteins to interact in elastic fibre assembly. In addition to their structural role, fibrillin microfibrils mediate cell signalling via integrin and syndecan receptors, and microfibrils sequester transforming growth factor (TGF)β family growth factors within the matrix to provide a tissue store which is critical for homeostasis and remodelling.

Recent updates on the molecular network of elastic fiber formation

Elastic fibers confer elasticity and recoiling to tissues and organs and play an essential role in induction of biochemical responses in a cell against mechanical forces derived from the microenvironment. The core component of elastic fibers is elastin (ELN), which is secreted as the monomer tropoelastin from elastogenic cells, and undergoes self-aggregation, cross-linking and deposition on to microfibrils, and assemble into insoluble ELN polymers. For elastic fibers to form, a microfibril scaffold (primarily formed by fibrillin-1 (FBN1)) is required. Numerous elastic fiber-associated proteins are involved in each step of elastogenesis and they instruct and/or facilitate the elastogenesis processes. In this review, we designated five proteins as key molecules in elastic fiber formation, including ELN, FBN1, fibulin-4 (FBLN4), fibulin-5 (FBLN5), and latent TGFβ-binding protein-4 (LTBP4). ELN and FBN1 serve as building blocks for elastic fibers. FBLN5, FBLN4 and LTBP4 have been demonstrated to play crucial roles in elastogenesis through knockout studies in mice. Using these molecules as a platform and expanding the elastic fiber network through the generation of an interactome map, we provide a concise review of elastogenesis with a recent update as well as discuss various biological functions of elastic fiber-associated proteins beyond elastogenesis in vivo.

Alternative splicing events during adipogenesis from hMSCs

Adipogenesis, the developmental process of progenitor-cell differentiating into adipocytes, leads to fat metabolic disorders. Alternative splicing (AS), a ubiquitous regulatory mechanism of gene expression, allows the generation of more than one unique messenger RNA (mRNA) species from a single gene. Till now, alternative splicing events during adipogenesis from human mesenchymal stem cells (hMSCs) are not yet fully elucidated. We performed RNA-Seq coupled with bioinformatics analysis to identify the differentially expressed AS genes and events during adipogenesis from hMSCs. A global survey separately identified 1262, 1181, 1167, and 1227 ASE involved in the most common types of AS including cassette exon, alt3, and alt5, especially with cassette exon the most prevalent, at 7, 14, 21, and 28 days during adipogenesis. Interestingly, 122 differentially expressed ASE referred to 118 genes, and the three genes including ACTN1 (alt3 and cassette), LRP1 (alt3 and alt5), and LTBP4 (cassette, cassette_multi, and unknown), appeared in multiple AS types of ASE during adipogenesis. Except for all the identified ASE of LRP1 occurred in the extracellular topological domain, alt3 (84) in transmembrane domain significantly differentially expressed was the potential key event during adipogenesis. Overall, we have, for the first time, conducted the global transcriptional profiling during adipogenesis of hMSCs to identify differentially expressed ASE and ASE-related genes. This finding would provide extensive ASE as the regulator of adipogenesis and the potential targets for future molecular research into adipogenesis-related metabolic disorders.

Clinical and molecular characterization of an 18-month-old infant with autosomal recessive cutis laxa type 1C due to a novel LTBP4 pathogenic variant, and literature review

BACKGROUND

Cutis laxa (CL) is a group of rare connective tissue disorders mainly characterized by wrinkled, redundant, inelastic, and sagging skin. Besides skin anomalies, in most CL forms multiple organs are involved, leading to severe multisystem disorders involving skeletal, cardiovascular, pulmonary, and central nervous systems. CL might be challenging to diagnose because of its different inheritance patterns, extensive phenotypic variability, and genetic heterogeneity. Herein, we report the clinical and molecular characterization of an 18-month-old infant with signs suggestive of recessive cutis laxa type 1C (ARCL1C), although with a relatively mild presentation.

METHODS

To confirm the clinical suspicion, mutational screening of all the exons and intron-flanking regions of the latent transforming growth factor-beta binding protein 4 gene (LTBP4) was performed by Sanger sequencing on an ABI3130XL Genetic Analyzer.

RESULTS

Apart from the presence of the dermatological hallmark, the reported patient did not show pulmonary emphysema, which is the most common and discriminative finding of ARCL1C together with gastrointestinal and urinary involvement. Indeed, pulmonary involvement only included episodes of respiratory distress and diaphragmatic eventration; intestinal dilation and tortuosity and hydronephrosis were also present. Molecular analysis disclosed the novel homozygous c.1450del (p.Arg484Glyfs*290) pathogenic variant in exon 12 of LTBP4, thus leading to the diagnosis of ARCL1C.

CONCLUSION

Our findings expand both the knowledge of the clinical phenotype and the allelic repertoire of ARCL1C. The comparison of the patient's features with those of the other patients reported up to now offers future perspectives for clinical research in this field.

Fibrillin microfibrils and elastic fibre proteins: Functional interactions and extracellular regulation of growth factors

Fibrillin microfibrils are extensible polymers that endow connective tissues with long-range elasticity and have widespread distributions in both elastic and non-elastic tissues. They act as a template for elastin deposition during elastic fibre formation and are essential for maintaining the integrity of tissues such as blood vessels, lung, skin and ocular ligaments. A reduction in fibrillin is seen in tissues in vascular ageing, chronic obstructive pulmonary disease, skin ageing and UV induced skin damage, and age-related vision deterioration. Most mutations in fibrillin cause Marfan syndrome, a genetic disease characterised by overgrowth of the long bones and other skeletal abnormalities with cardiovascular and eye defects. However, mutations in fibrillin and fibrillin-binding proteins can also cause short-stature pathologies. All of these diseases have been linked to dysregulated growth factor signalling which forms a major functional role for fibrillin.

LTBPs in biology and medicine: LTBP diseases

The latent transforming growth factor (TGF) β binding proteins (LTBP) are crucial mediators of TGFβ function, as they control growth factor secretion, matrix deposition, presentation and activation. Deficiencies in specific LTBP isoforms yield discrete phenotypes representing defects in bone, lung and cardiovascular development mediated by loss of TGFβ signaling. Additional phenotypes represent loss of unique TGFβ-independent features of LTBP effects on elastogenesis and microfibril assembly. Thus, the LTBPs act as sensors for the regulation of both growth factor activity and matrix function.